gh-36964: Add `# sage_setup: distribution` directives to all files, remove remaining `# coding: utf-8`
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These directives at the top of the file inform developers about the
intended assignment of modules to pip-installable distributions.
As of this PR, there should be no change to the existing distributions
(sagemath-categories...) nor the monolithic build of the Sage library.
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- Cherry-picked from #35095
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### :hourglass: Dependencies
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- Depends on #36951 (merged here)
- Depends on #36676 (merged here)
- Depends on #37667 (merged here)
URL: https://github.com/sagemath/sage/pull/36964
Reported by: Matthias Köppe
Reviewer(s): Kwankyu Lee, Matthias Köppe, Tobias Diez
